Embedded CSS Framework
Blueprint ?

I am also using bootstrap. Can't say if it is the best cause I haven't tried all but will probably try Foundation soon. But generally I am pretty happy with B haven't had major hurdles. The only drawback that bootstrap has is it doesn't have speech to text so I don't even have to type my layout

Althoug I like bootstrap and use it in almost any project I dont think it has to be part of the framework. May be released under some official extension but not part of the core fw. This just bounds the fw too much to something that may not be so good in the near future.